Allow assigning GCP SA in databricks_sql_global_config resource

[zjiaksmc]

Changes

Allow google_service_account to be assigned to SQL warehouse global settings.
This change impacts GCP Databricks workspace only. Corresponding REST API endpoint

Tests

• make test run locally
• relevant change in docs/ folder
• covered with integration tests in internal/acceptance
• relevant acceptance tests are passing
• using Go SDK

[alexott]

Tests most probably will fail. Also, update the docs

[alexott]

also, run make fmt

[alexott]

you can add a separate test where fixture is create with Gcp: true

[zjiaksmc]

@alexott I just incorporated all your suggestions.

[alexott]

looks good, although it would be nice to add a test for this branch

else {
  return fmt.Errorf("can't use google_service_account outside of GCP")
}